1. Field of the Invention
The present invention relates to a storage apparatus, storage control circuit, and head-position-displacement measuring method for detecting a head position displacement of a write head and a read head provided to a head and correcting an amount of position displacement of the read head at the time of reading a storage medium. In particular, the present invention relates to a storage apparatus, storage control circuit, and head-position-displacement measuring method for measuring a head position displacement targeted for a storage medium provided to a non-magnetic area between tracks.
2. Description of the Related Arts
In magnetic disk apparatus in recent years, a combined-type head structure has been adopted in which a highly-sensitive read head using, for example, tunnel magnetoresistance effect (TMR), is independent from a write head. In the head having such a configuration, a position displacement of the write head and the read head occurring in a head manufacturing process cannot be completely eliminated, and therefore paths through which the write head and the read head pass at the time of recording on a certain track will be different. Therefore, it is required that such a displacement amount be measured in advance in, for example, a manufacturing inspection process, for storage, and then a correction be made by that displacement amount from a radial position at the time of recording data on a target track to read-back the data on the target track. In recent years, a space between adjacent tracks has been on the order of 200 nm. If a position displacement of the write head and the read head is several μm, a position displacement correction corresponding to several tracks has to be required. This tendency becomes a more important problem as the space between tracks is narrowed more with the advance of high density. Also in recent years, in magnetic disk apparatus, the tide of downsizing is significant and, accordingly, a disk medium capable of high-density recording has been demanded. However, in improving the recording density of magnetic disk s, there is a problem of how to prevent interference from an adjacent recording bit. Under the recognition of such a problem, a technology called discrete-track recording has been conventionally suggested, in which, against interference of a magnetic disk medium in a radial direction, the magnetic disk medium is physically sectioned for each track to reduce interference from an adjacent track. Furthermore, under the recognition of the same problem in which interference from an adjacent recording bit is prevented to address an increase in recording density, a technology called patterned media has also been conventionally suggested in which a magnetic disk medium is physically sectioned also in a perimeter direction, that is, patterning in units of recording bits is performed, thereby reducing interference from an adjacent bit. Therefore, it has been demanded that, also for storage media using technologies, such as discrete tracks and patterned media, a position displacement correction of the write head and the read head be accurately performed to achieve high-density recording.
FIG. 1 linearly depicts one track length of a recording medium in a process of measuring a head position displacement, in which M number of servo sectors correspond to one cycle of track. To correct a position displacement of a write head 202 and a read head 204 provided to a combined-type head 200, first with the write head 202 being positioned at a track 204-N at a certain radial position by using a head position signal obtained through servo-sector reading by the read head 204, a measurement signal of one cycle of track is recorded on a measurement pattern 206 as shown. At this time, since a position displacement is present between the read head 204 and the write head 202, the read head 204 takes a path 208 that matches with a track 204-N, but the write head 202 takes a path 210 at a position shifted from the track 204-N, the position where the measurement pattern 206 is written. The amount of position displacement is detected by, after writing the measurement pattern 206, reading a read position in a radial direction at which the read head 204 is positioned while gradually shifting the read position of the read head 204 from the position where the measurement pattern 206 is written, and finding, for example, a profile (distribution) of amplitudes of a read-back signal, thereby obtaining FIG. 2A.
In FIG. 2A, a profile (signal distribution) can be obtained in which the amplitude of the read-back signal is low at a write position 212 of the measurement pattern, the amplitude of the read-back signal is increased when the measurement pattern is read while gradually shifting the read position and, after reaching its peak point 214, the amplitude is again decreased. An offset value 218 to the peak point 214 at which the amplitude of the read-back signal is maximum in the profile 216 of the read-back signal amplitude 216 is detected as an amount of position displacement of the read head and the write head. Here, the value of the amplitude of FIG. 2A represents an average value of amplitudes of the read-back signal read over one track. Amplitudes of the read-back signal of the peak 214 and a ½ peak point 220 are average values of the read-back signal amplitudes 222 and 224 read over one track as depicted in FIG. 2B. For example, a method has been suggested in which, by using this position displacement detection method, an area for measuring a track displacement is provided on the storage medium to measure changes with time in position displacement of the write head and the read head (JP 09-045025). However, in such a position displacement detection method, it is impossible to support discrete tracks and patterned media that may possibly be positioned at a place where no magnetic substance is placed. For example, as depicted in FIG. 3, in the case of discrete tracks, a non-magnetic area 228 is present on both sides of a track formed of a magnetic area 226. If a path 210 of the write head 202 when the read head 204 is positioned on the track 204-N is just on the magnetic area 226, a profile when the read head 204 is offset to position displacement detection to search the measurement pattern 206 is similar to that of FIG. 2A. However, as depicted in FIG. 4, when the path 210 of the write head 202 is on the non-magnetic area 228, the measurement pattern 206 is not accurately recorded, and a profile of amplitudes of the read-back signal when the measurement pattern 206 is read while gradually shifting the read position of the read head 204 is such as a profile 216-1 of FIG. 5A, thereby making it difficult to specify an offset value of a peak point 214-1 where the amplitude of the read-back signal is at maximum. Here, FIG. 5B represents amplitudes of the read-back signal at read positions of the peak point 214-1 and the ½ peak point 220-1 as read-back signal amplitudes 222-1 and 224-1 read over one track. To solve these problems, a storage medium and a magnetic storage apparatus have also been suggested in which an area in which magnetic substances are successively formed in a radial direction of the medium is provided between a servo sector and a data sector of a storage medium (JP 2005-166115 and JP 2005-166116).
However, when the area in which magnetic substances are successively formed in the radial direction of the medium is provided between the servo sector and the data sector as in JP 2005-166115 is used for a special area for position displacement correction measurement of the write head and the read head, user data cannot be written in that area, thereby decreasing the recording capacity by that amount. Moreover, in order to accurately measure the amount of position displacement correction, a measurement area that is long to some extent, for example, one cycle of track, is required so as to average read-back signals of the measurement pattern to remove noise, thereby causing user data format efficiency to deteriorate.